1. Field of the Invention
This invention relates to measuring instruments wherein a measuring element support member is made movable on guide rails, such as a two-dimensional measuring instrument, three-dimensional measuring instrument and contour measuring instrument, and more particularly to a scale protection device affixed to the guide rail.
2. Description of the Prior Art
In general, in the measuring instrument of the type described, in order to measure the displacement value of the measuring element support member with high accuracies, it is necessary to use scales finished with high accuracies, such as a glass scale, magnetic scale, reflecting type metallic plate scale and the like. Further, in order to maintain this high extent of accuracy, it is necessary to protect the outer surface of the scale. More particularly, if there occur adhesion of oil through contact by operators, accumulation of iron scraps, dust and the like and damages caused by abutted tools and the like, a desirable measuring accuracy cannot be attained.
In view of the above, in the measuring instruments of the prior art, protection devices have been provided for the surface of the scales. However, the measuring instruments of the prior art have presented such disadvantages that the measuring accuracy is lowered, difficulties are felt in assembling and disassembling and so forth due to increased sliding resistance. For example, according to Japanese Utility Model Applications Publication Nos. 32506/1964 and 35765/1981, there is described such a construction that a single belt-like member is stretchingly provided in the longitudinal direction of a rack or a scale, this belt-like member extending through a slidable bed or a slider, movable relative to a scale or the like, and this belt-like member is closely attached to the surface of a scale containing case or the surface of a scale under tension during stretching. However, since, in these well known constructions, the belt-like member is used integrally with the case or scale, it is necessary to increase the tension in order to strengthen the attaching force. The increased attaching force increases the sliding resistance of the slider excessively, and has contributed to the occurrence of errors in returning at the time of reversing the moving direction. Furthermore, the thickness of the slider and the like in a direction perpendicular to the plate thickness of the belt-like member has been increased, whereby the device has been rendered large sized and the construction has been complicated, thus presenting the disadvantages that difficulties are felt not only in assembling and disassembling but also in inspecting the scale. Further, there have been raised problems that there are possibilities of a gap occurring between the belt-like member and the case due to expansion or shrinkage caused by a temperature change in the belt-like member, separation caused by vibrations and the like which effects looseness of the belt-like member and a hindering of the slider in its movement.